switch to yeslogic-fontconfig-sys instead of servo-fontconfig

servo-fontconfig statically links a vendored fontconfig library if fontconfig is not found by pkgconfig. Using a vendored fontconfig
library instead of the system fontconfig library doesn't actually work well though:
https://github.com/slint-ui/slint/issues/88
Building a vendored copy of fontconfig is also problematic because fontconfig has a lot of C dependencies, which makes it difficult to cross compile the vendored copy of fontconfig:

```
$ pkg-config fontconfig --static --libs
-lfontconfig -lfreetype -lz -lbz2 -lpng16 -lm -lm -lz -lharfbuzz -lm -lglib-2.0 -lm -lpcre -lsysprof-capture-4 -pthread -lgraphite2 -lbrotlidec -lbrotlicommon -lxml2 -lz -llzma -lm
```

Instead of using a vendored copy, with
https://github.com/yeslogic/fontconfig-rs/pull/12
yeslogic-fontconfig-sys will have a Cargo feature to dlopen fontconfig at runtime instead of linking it at build time. This is exposed in font-kit with the new source-fontconfig-dlopen feature, which is disabled by default. This feature makes it considerably easier to cross compile by avoiding the need to cross compile fontconfig and all its dependencies.